Different turbine characteristics are required
for each type of application. Most electrical-power generators, or alternators,
tend to require a relatively high speed of rotation. Consequently, turbines
that run fairly fast, with the tips of the rotor moving 3-10 times the wind
speed, are generally preferable to low-speed turbines because less gearing is
required between the shaft of the wind turbine and the armature of the
generator. A relatively low running speed is generally favoured for wind
turbines that are directly coupled to water pumps or other mechanical loads. A
striking feature of wind turbines intended for high-speed operation is the low
rotor solidity, that is, the very small blade area in proportion to the turbine
rotor's total projected area. low-speed turbines feature a high solidity
(either a small number of relatively broad blades or a large number of narrower
ones).

All modern high-speed turbines and
most of the low-speed units incorporate blades designed on airfoil principles.
Some inefficient low-speed machines depend on a drag effect, as does a
square-rigged sailing ship. The foregoing remarks apply equally to wind
turbines that are arranged with horizontal or vertical axes of rotation.
Typical horizontal-axis high-speed wind turbines have either 2 or 3 blades and
resemble aircraft propellers. The blades of these machines are commonly
arranged to vary in pitch automatically to optimize performance under
conditions of varying wind speed. The most common high-speed vertical-axis
turbine is the Darrieus rotor, named for its inventor. This machine is also
known as an "egg beater" because of its characteristic appearance.

The most common form of low-speed
machine is the horizontal axis, multi bladed form often found on farms. The
turbine is usually connected, via a crank, to a reciprocating water pump. A
related design is the multi bladed "bicycle-wheel" turbine, an example
of a relatively low-speed turbine used for electrical-power generation. A
simple vertical-axis design, often used for water pumping, is the split
cylinder configuration known as the Savonius rotor, after its originator. In
this turbine each of the 2 or 3 rotor blades consists of a semi cylinder offset
radially from the axis of rotation. The design relies, in part, on a drag
effect for its operation. It is not, therefore, a particularly efficient
configuration, but is relatively simple to make.

General advantages of wind
turbines are the complete absence of air pollution and a high energy-conversion
efficiency. Well-designed wind turbines can recover up to 60- 80% of the
kinetic energy from the flow passing through their rotors. However, the low
energy density available in the wind typically restricts output to a range 0.1
to 0.8 kW/m2 (kilowatts per square metre) of the rotor's projected area. The
result is a large machine size in relation to output; for example, in a machine
rated at 5 MW (MW=106 W), a high output for a single wind turbine, the rotor
diameter can be as large as 100 m. This size problem results in the use of
clustered machines, known as wind farms, to extract large power outputs from
individual sites. Another problem with wind-energy systems is the variability
of wind strength, which leads to substantial fluctuations in power since output
is roughly proportional to the cube of the wind speed. Furthermore, all
wind-energy conversion devices incur additional costs because they must be capable
of withstanding storms.

Savonius

is a type of vertical
axis wind turbine (VAWT)
generator. The Savonius is adrag-typeVAWT
which operates in the same way as a cup anemometer Savonius wind turbines
typically only have an efficiency of around 15% - i.e. just 15% of the wind
energy hitting the rotor is turned into rotational mechanical energy. This is
much less than can be achieved with a Darrieus wind turbine (which usesliftrather
than drag.

The
speed of the cups
of a cup anemometer (and aSavonius wind turbine) cannot rotate faster
than the speed of the wind and so they have atip
speed ratio(TSR) of 1 or below. Therefore Savonius type vertical axis wind
turbines turn slowly but generate a high torque.

This
does
not make them very suitable forelectricity
generationsince turbine
generators need to be turned at hundreds of RPM to generate high voltages and
currents. A gearbox could be employed but the added resistence would leave the
Savonius requiring a very strong wind to get spinning - i.e. it typically would
notself-start.

Savonius
wind turbines

are ideally suited to applications such as pumping water
and grinding grain for which slow rotation and high torque are essential.
Because of the torque yield of a Savonius wind turbine, the bearings used must
be very sturdy and may require servicing every couple of years

A
Darrieus wind turbine
can spin at many times the speed of the wind hitting it (i.e. thetip speed
ratio(TSR) is greater than 1). Hence a
Darrieus wind turbine generates less torque than a Savonius but it rotates much
faster. This makes Darrieus wind turbines much better suited to electricity
generation rather than water pumping and similar activities. The centrifugal
forces generated by a Darrieus turbine are very large and act on the turbine
blades which therefore have to be very strong - however the forces on the
bearings and generator are usually lower than are the case with a Savonius.

Darrieus
wind turbines are notself-starting.
Therefore a small powered motor is required to start off the rotation, and then
when it has enough speed the wind passing across the aerofoil’s starts to
generate torque and the rotor is driven around by the wind.

An alternative is shown in the illustration
above. Two small Savoniusrotors
are mounted on
the shaft of the Darrieus turbine to start rotation. These slow down the
Darrieus turbine when it gets going however they make the whole device a lot
simpler and easier to maintain